Catalyst poisons, pyridine

Wlien a strong electron-donor ligand such as pyridine is added to tlie reaction mixture, it can bond so strongly to tlie Rli tliat it essentially drains off all tlie Rli and shuts down tlie cycle it is called a catalyst poison. A poison for many catalysts is CO it works as a physiological poison in essentially the same way as it works as a catalyst poison it bonds to tlie iron sites of haemoglobin in competition witli O. ... 

Reductions with noble metal catalysts proceed smoothly (at 20°C) when the bases are in the form of hydrochlorides the free bases tend to poison the catalyst. A pyridine ring is reduced more easily than a benzene ring thus, 2-phenylpyridine gives 2-phenylpiperidine (384), quinoline gives 1,2,3,4-tetrahydroquinoline (385) and acridine gives 9,10-dihydroacridine (386). 

Special care has to be taken, however, that the quinoline titer truly represents the minimum amount of catalyst poison. In most cases this type of base is adsorbed by inactive as well as active sites. Demonstration of indiscriminate adsorption is furnished by the titration results of Roman-ovskii et al. (52). These authors (Fig. 13) showed that introduction of a given dose of quinoline at 430°C in a stream of carrier gas caused the activity of Y-zeolite catalyst (as measured by cumene conversion) to drop with time, reach a minimum value, then slowly rise as quinoline was desorbed. The decrease in catalytic activity with time is direct evidence for the redistribution of initially adsorbed quinoline from inactive to active centers. We have observed similar behavior in carrying out catalytic titrations of amorphous and crystalline aluminosilicates with pyridine, quinoline, and lutidine isomers. In most cases, we found that the poisoning effectiveness of a given amine can be increased either by lengthening the time interval between pulse additions or by raising the sample temperature for a few minutes after each pulse addition. 

Prereduced rhenium heptoxide catalyst,65 especially the catalyst poisoned with pyridine, has been found to give high yields of unsaturated alcohols in the hydrogenation of unsaturated aldehydes (Table 5.2).66 A typical hydrogenation with the rhenium catalyst is shown in eq. 5.27. In the vapor phase hydrogenation of acrolein to allyl alcohol, the selectivity of rhenium catalysts has been found to be improved by poisoning with CO and CS2.67... 

The steric properties of the pyridine substrate are critical. Pyridines which lack ortho substituents form nonlabile, unreactive 18-electron pyri-dyl pyridine complexes 36 [see also Eq. (37)]. In fact, one of the principal catalyst deactivation processes in a-picoline coupling reactions is catalyst poisoning via formation of 36 by trace amounts of 3- and 4-methylpyridine impurities in the a-picoline feed. 

In the final deactivation mode reported by the authors, the active acidic sites of the catalyst are poisoned (7 = 145°C, P = 50 bar) by continuous addition of a very dilute solution of pyridine to the reacting mixture over a period of 12 h (see figure 11.10). The catalyst can be reactivated by heating and compressing the reaction mixture to conditions well within the mixture critical region (7 = 250°C, P = 500 bar). Tiltscher and coworkers report that the catalyst poison is precipitated from the product solution as pyridinium chloride. Presumably only a very small amount of pyridinium chloride is needed to deactivate the catalyst since supercritical hexene probably would not be able to solubilize much of this salt. It is surprising, however, that supercritical hexene can overcome the acid-base interactions that are occurring on the catalyst surface and, hence, remove the pyridinium chloride. 

The plot represents the relative optical yields, eekco, versus the relative reaction rates, v/Vq, where and eCo (11%) are the rate and ee for the unpoisoned catalyst. Poisons are butylamine (V), triethylamine (o), ammonia ( ) or pyridine (A) (mainly according to Yasumori ). 

NiMo/CB catalysts were 0.84 and 0.28, respectively. This resulted from poisoning of the HDS sites on the NiMo/AhOs catalyst by pyridine and N-containing intermediates. The diminished poisoning was the reason for a high HDS rate and low HDN/HDS ratio on the NiMo/CB catalyst. The experiments were conducted in a continuous system at 593 K and 2 MPa of H2. 

Most of them are generally classified as poisons. Exceptions to this rule are known. A notable one is 4-dimethyl aminopyridine (DMAP) (24), which is widely used in industry as a superior acylation catalyst (27). Quaternary salts of pyridines are usually toxic, and in particular paraquat (20) exposure can have fatal consequences. Some chloropyridines, especially polychlorinated ones, should be handled with extra care because of their potential mutagenic effects. Vinylpyridines are corrosive to the skin, and can act as a sensitizer for some susceptible individuals. Niacin (27), niacinamide (26), and some pyridinecarbaldehydes can cause skin flushing. 

The discussion in the previous section suggests that adsorption of pyridine on the catalyst is a necessary prerequisite for the formation of 2,2 -bipyridine but as platinum catalysts, which are poisoned by... 

Adsorbed CO and NO were used as probes to Investigate the effects of Co concentration and sulfide on the nature and numbers of exposed metal sites on reduced catalysts containing 1 to 6 wt% Co and 8 wt%. Mo on three alumina supports. Exposure of Mo Ions decreased with Increased Co concentration. Exposure of Co Ions typically reached a maximum at 2-4% Co. Sulfide decreased exposure of all metal Ion sites and Increased exposure of reduced metals. Effects of preadsorbed pyridine and 2,6-lutldlne, known poisons, on the exposure of metal sites, plus other evidence. 

Other poisons (modifiers) used to create such selective Pd catalysts may be metals 23 Zn, Cd, Zr, Ru, Au, Cu, Fe, Hg, Ag, Pb, Sb, and Sn or solvents (organic modifiers) 24 pyridine, quinoline, piperidine, aniline, diethylamine, other amines, chlorobenzene, and sulfur compounds. Hydroxides have also been used to increase the half-hydrogenation selectivity of Pd. 

The selective reduction of the D-ring olefin in 106 using a partially poisoned catalyst (Pd/C, 0.25 % pyridine) provided intermediate 107 (83 %), which was epimerized at -78 °C with sodium methoxide (HOAc quench at -78 °C, 89 %) (Scheme 10.9). Deoxygenation by means of tosyl hydrazone 108 and subsequent treatment with catechol borane and tetrabutylammonium acetate gave pentacyclic... 

Fig. 3. Correlation of the slopes p for the dehydration of secondary alcohols on various catalysts (series 3-6) with independently measured heats of adsorption of water and diethyl ether, sensitivity to pyridine poisoning (41), and deuterium kinetic isotope effects (68). [Reprinted with permission from Berdnek and Kraus (13, p. 294). Courtesy Elsevier Scientific Company.]...

Pyridinium salts can be reduced more easily than the corresponding pyridines, and yields are higher because the resulting N-methylpiperidines do not poison the catalyst. 

---